The availability of CO gas that is as pure as possible is highly desirable for many chemical syntheses. CO gases are often contaminated with impurities, for example sulfur compounds, which act as catalytic poisons or secondary-reaction initiators or which lead in other ways to interruptions in operation or even to the total loss of production.
Many attempts have therefore been made to obtain pure forms of preparation of that gas.
For example, the method of converting organic sulfur compounds in the presence of steam on an aluminum oxide contact at temperatures of about 160° C. to form hydrogen sulfide and carbon dioxide is described on p. 3, line 6 ff of DE-A 43 21 542, although detailed information regarding the degree of conversion of the organic sulfur compounds is not given.
DE-A 41 04 202 describes on p. 3, line 14 ff, the almost quantitative hydrolysis of carbon oxide sulfide at markedly lower temperatures of from 20 to 150° C. on aluminum oxide contacts which, however, have been doped with further very expensive transition metals. Column 2, line 9 ff and column 7, line 20 ff of EP-A 0 324 526 describe the use of a titanium dioxide contact for the hydrolysis of COS and CS2 at temperatures above 300° C. with a lower degree of conversion of from 90 to 96% (based on organic sulfur compounds). However, those processes use contacts which are expensive and complex to prepare using doped aluminum oxides or titanium dioxide.
In GB-A 22 48 444, there is described the use of an aluminum oxide contact for the conversion of hydrogen sulfide with carbon dioxide at from 60 to 120° C. to carbon oxide sulfide and water as a reverse reaction; the resulting carbon oxide sulfide is subsequently decomposed thermally. EP-A 0 234 433 describes the use of an aluminum oxide contact doped with alkali for the absorption (not the conversion) of acidic gases, inter alia hydrogen sulfide. However, these processes have the disadvantage that the Al2O3 contacts used either have to be pretreated in a complex and expensive manner or effect too low a degree of conversion or require the use of too great an amount of energy.
The process of the chemical absorption of hydrogen sulfide on iron-hydroxide-containing materials with the concomitant use of oxygen has already been mentioned in DE-A 20 06 758; the optimization of the absorption materials in respect of the content of iron hydroxide and loosening agent is described therein on p. 2. DE-A 28 45 725 claims the preparation of iron-hydroxide-containing absorption materials based substantially on iron (II) sulfate and calcium oxide and their use in the chemical binding of hydrogen sulfide. DD-A 15 72 62 mentions on p. 3 the use of limonite as an iron-hydroxide-containing material for the removal of hydrogen sulfide from CO-containing gases.
U.S. Pat. No. 6,126,911 describes the use of iron oxides (claim 3) or iron oxide hydrates (column 5, line 18) in combination with moistening agents such as glycols etc. for the absorption of hydrogen sulfide from dry gases, with only 67% of the hydrogen sulfide contained in the gas stream being retained in the most advantageous case (column 7, line 55). WO-A 93/13184 describes the absorption of hydrogen sulfide from dry gases on iron oxides onto SiO2-containing carrier materials in the presence of oxygen at considerably higher temperatures of about 200° C. (p. 22, line 20 ff to p. 23, line 22). The hydrogen sulfide is thereby retained on the contact in the form of elemental sulfur with an efficiency of only 80%. After catalytic hydrogenation of the gas mixture for the complete conversion of sulfur compounds into hydrogen sulfide, the residual amounts of sulfur are adsorbed on activated carbon doped with iron oxide. Column 6, lines 16-40 of EP-A 1180544 describes the use of tri-iron tetra-oxide at 400° C. for the absorption of hydrogen sulfide on the contact in the form of iron sulfide; in a subsequent step, residual amounts of hydrogen sulfide are bonded to zinc oxide in the form of zinc sulfide. The examples of EP-A 722 774 describe the use of iron(III) oxide in combination with molybdenum oxide on an aluminium oxide carrier at least 350° C. for the absorption of hydrogen sulfide in the form of sulfide on the contact. However, all those processes either use undesirable auxiliary substances (glycol) or require the use of far too much energy, or technically complicated subsequent steps (deposition of sulfur), to achieve usable results.
The process of the physical adsorption of gases on activated carbon is generally known in the art. For the adsorption of hydrogen sulfide from CO gas, DE-A 4321542 claims an activated carbon that has been doped with iodine or iron compounds and permits a residual content of hydrogen sulfide in the CO gas of less than 10 mg/m3. WO-A 93/13184 describes (p. 23, line 12 ff) the use of an iron-oxide-containing activated carbon on which hydrogen sulfide is oxidized to elemental sulfur in the presence of oxygen at 50° C. and adsorbed to a residual content of 2 vol.ppm hydrogen sulfide in the CO gas. However, pretreatment of the activated carbon is necessary in those processes, and in some cases excessively high absorption temperatures.